We were 85 feet (26 meters) underwater and more than five miles (eight kilometers) off the Florida coast when the lights went out. It was night, and conservationist Craig Taylor and I had been diving for two hours by the dim beams of the Aquarius research station—our underwater home and, at that depth, our only safe haven. About the size of a railroad freight car, Aquarius looked like a spaceship on the seafloor, an interior glow filling her view ports, exterior spotlights illuminating her sides and legs. When she lost power, she simply disappeared into the inky blackness, and I felt as cut off from the world as an astronaut stranded in space.

I fought the impulse to head for the surface, which is what scuba divers are trained to do when they get into trouble, because this was no ordinary dive. For the past four days we'd been living in Aquarius as aquanauts, and by now our bodies were saturated with nitrogen. If I surfaced quickly, without decompression, dissolved nitrogen in my body would expand from the sharp decrease in pressure, forming bubbles that could painfully squeeze nerves, block blood flow, or cause brain damage. Decompression sickness probably would kill me.

Suddenly Aquarius's emergency siren started wailing—a signal for all aquanauts to return immediately. The piercing sound, however, seemed to come from all directions. Breathing heavily on my scuba tanks as I swam through the darkness, I used my emergency lights to search for the web of excursion lines that had been mapped out for us during our one-week training session. These guidelines were a safety measure to help us navigate around the reef. Grasping a black braided rope in one gloved hand, Craig and I followed the line back to the station, where we felt our way across the coral-and-algae-covered metal to the rectangular opening in the bottom known as the moon pool.

The station functions like an inverted glass pushed down into a bucket of water: An air pocket remains at the top of the glass while the glass remains upright. The crew maintains the air pressure inside Aquarius at the same high pressure as the surrounding ocean, keeping the water from rushing in. We lived in that air pocket, which I was eager to get back to. Emerging from the ocean water, I stood waist-deep in the moon pool, removed my regulator, and breathed in the hot, humid air from Aquarius.

"Generator's down," said Christian Petersen, a U.S. Navy diving medical officer, as he stood above me in the dim emergency lighting. Without power from either of the two electric generators in the life-support buoy tethered above us on the surface, we had only dim emergency lights and no air-conditioning. In these warm tropical waters, with a half dozen people inside, our small laboratory would rapidly become stifling. I climbed up the stainless steel steps, peeled off my dive gear, and began to sweat.

to January 1945 when photographer Lilo Hess lured a striped burrfish to a microphone to hear what it had to say.

In More to Explore the National Geographic magazine team shares some of its best sources and other information. Special thanks to the Research Division.

A graphic way to visualize what can happen in a diver's body if the pressure of the surroundings is reduced too suddenly is to recall what happens in a bottle of soda when the top is removed quickly. The pressure that held the gas—carbon dioxide in soft drinks—in solution suddenly drops and the gas comes out of the solution in a burst of bubbles, flowing out over the top of the bottle.

Such a sudden release of pressure isn't only a hazard for divers, but also can be a danger in unpressurized civilian aircraft and high-flying military planes. Our bodies are saturated with nitrogen all the time, no matter the altitude at which we live. If an unpressurized airplane (usually a small, privately owned plane) rises quickly to altitudes of more than about 18,000 feet (5,500 meters) above sea level, that dissolved nitrogen can form bubbles, and the pilot and passengers suddenly find themselves dealing with the bends, even though they may be breathing oxygen to compensate for the thinner atmosphere. The solution is, of course, to come down to a lower altitude. Altitude-induced decompression sickness can also threaten military personnel who sometimes must work in depressurized sections of aircraft for a while, such as when a cargo door is opened for parachutists or an airdrop. In these instances, personnel who will be working in the unpressurized area breathe pure oxygen for a carefully regulated time period before the plane rises above a dangerous altitude, just as the aquanauts in Aquarius spent three 20-minute periods breathing oxygen at the beginning of their decompression. In each case, whether the person is high above the Earth or beneath the ocean's surface, the oxygen forces some of the dissolved nitrogen out of the body and reduces the chance of getting decompression sickness.

—Patricia Kellogg

Aquariuswww.uncw.edu/aquariusAt the Aquarius website you'll find technical details about the habitat and past missions, and when a mission is under way take a virtual visit through a live webcam. The site includes lesson plans, links, and lots of photographs.

National Marine Sanctuary Programwww.sanctuaries.nos.noaa.govHere you will find background on the national program and links to the websites of the 13 marine sanctuaries. Good resources for teachers, students, and anyone interested in oceanography.

New England Aquariumwww.neaq.orgTake a tour of Boston's New England Aquarium, investigate the research and conservation programs it's involved in, and find dozens of links.

Office of Naval Research Science and Technology Focuswww.onr.navy.mil/focusThis is a great place to begin an investigation into almost any aspect of oceanography. It also offers links, lessons, and a place to submit questions to ONR's scientists.